Attention has been paid to the use of an excimer laser as a light source of reduction projection aligner (hereinafter called a stepper) for manufacturing semiconductor devices. This is because the excimer laser may possibly extend the light exposure limit to be less than (0.5 .mu.m since the wavelength of the excimer laser is short (for example the wavelength of KrF laser is about 248.4 nm), because with the same resolution, the focal depth is greater than a g line or an i line of a mercury lamp conventionally used, because the numerical aperture (NA) of a lens can be made small so that the exposure region can be enlarged and large power can be obtained, and because many other advantages can be expected.
An excimer laser utilized as a light source of the stepper is required to have a narrow bandwidth with a beam width of less than 3 pm as well as a large output power.
A technique of narrowing the bandwidth of the excimer laser beam is known as the injection lock method. In the injection lock method, wavelength selecting elements (etalon, diffraction grating, prism, etc.) are disposed in a cavity of an oscillation stage so as to generate a single mode oscillation by limiting the space mode by using a pin hole and to injection synchronize the laser beam in an amplification stage. With this method, however, although a relatively large output power can be obtained, there are such defects that misshots occur, that it is difficult to obtain 100% locking efficiency, and the spectrum purity degrades. Furthermore, in this method, the output light beam has a high degree of coherency so that when the output light beam is used as a light source of the reduction type projection aligner, a speckle pattern generates. Generally it is considered that the generation of speckle pattern depends upon the number of space transverse modes. When the number of space transverse modes contained in the laser light is small, the speckle pattern becomes easy to generate. Conversely, when the number of the space transverse modes increases, the speckle pattern becomes difficult to generate art. The injection lock method described above is a technique for narrowing the bandwidth by greatly decreasing the number of space transverse modes. Since generation of speckle pattern causes a serious problem, this technique can not be adopted in the reduction projection aligner.
Another projection technique for narrowing the bandwidth of the excimer layer beam is a technique utilizing a air gap etalon acting as a wavelength selective element. A prior art technique utilizing the air gap etalon was developed by AT & T Bell Laboratory wherein an air gap etalon is disposed between the front mirror and a laser chamber of an excimer laser device so as to narrow the bandwidth of the excimer laser. This system, however, cannot obtain a very narrow spectral bandwidth. In addition there are problems in that the power loss is large due to the insertion of the air gap etalon. Further, it is impossible to greatly increase the number of the space transverse modes. Furthermore, the air gap etalon has a problem of poor durability.
Accordingly, an excimer laser device has been proposed wherein a relatively high durable diffraction grating is used as the wavelength selective element.
In the excimer laser having a diffractive grating which acts as a wavelength selective element, a pin hole is provided in a resonator (laser cavity) to reduce the spread angle of the beam in the grating. Alternatively, a beam expander is provided to expand the laser beam incident to the grating. For this beam expander, a prism expander utilizing a prism is typically been used.
The narrow band excimer laser used as the stepper must not only have a narrow bandwidth with a line width of less than 3 pm, but also must produce a large output.
In the construction in which a pin hole is disposed in a resonator, however, the output becomes greatly reduced and the number of space transverse modes necessary for preventing generation of a speckle pattern decreases, so that such construction can not be used.
In the construction in which a prism beam expander is used, the expander must have a large magnifying power in order to narrow the line width.
However, when the magnifying power of the prism beam expander becomes large, the incident angle of the laser beam to a prism of the prism expander becomes large or it becomes necessary to increase the number of prisms, thereby increasing the loss. As a result, a large output cannot be produced.
Furthermore, where a narrow band excimer laser is used as the light source of a stepper, it is necessary to narrow the bandwidth of the output laser beam and then to control the wavelength of the output laser beam whose bandwidth has been narrowed to a stable condition at a high accuracy.
A monitor etalon has been used for measuring the line width of the output beam and for detecting the wavelength. The monitor etalon is constituted by an air gap etalon wherein a pair of partial reflective mirrors are disposed to confront each other with a predetermined air gap there-between. The transmissive wavelength of this air gap etalon is expressed by the following equation EQU m.lambda.=2nd.multidot.cos .theta.
where m represents the order, d the partial mirrors spacing, n the refractive index of the medium the partial reflective mirrors, and .theta. an angle between the normal of the etalon and the axis of the incident light.
This equation shows that where n, d and m are constant, as the wavelength varies, the angle .theta. changes. The monitor etalon detects the wavelength of the beam by utilizing those characteristics. In the monitor etalon described above, when the pressure in the air gap and the ambient temperature vary, the angle .theta. varies even when the wavelength is constant. Accordingly, where the monitor etalon is used for detecting the wavelength, the pressure in the air gap and the ambient temperature are controlled to be constant.
However, it is difficult to precisely control the pressure in the air gap and the ambient temperature. Therefore, it is impossible to detect the absolute wavelength at a high accuracy.
For this reason, apparatus has been proposed wherein the beam to be detected is inputted to the monitor etalon together with a reference beam having a known wavelength, and the wavelength of the beam is detected by detecting a wavelength of the beam relative to the reference beam.
In this apparatus, a light beam transmitting through the monitor etalon is directly inputted to a beam detector such as CCD image sensor.
However, in this apparatus, since the output of the monitor etalon is directly inputted to the beam detector, the beam to be detected and the reference beam cannot be inputted to the beam detector with a sufficient beam quantity, and an interference fringe cannot be formed on the beam detector.
Accordingly, it is an object of this invention to provide a novel narrow band excimer laser of the type using a prism beam expander and a diffraction grating as a band-narrowing element capable of preventing increase of loss even when the magnifying power of the prism expander is increased.
Another object of this invention is to provide a novel wavelength detecting apparatus of a narrow band excimer laser capable of inputting a reference beam and a beam to be detected into a beam detector with a sufficient quantity and capable of detecting the interference fringes of both beams at a high accuracy.